Solar sizing
Battery, panel and controller for a small 12V system. Sized against the season you pick, which in Britain changes the answer by a factor of seven.
Measure it if you can. Watts times hours, for every load, added up.
Three is a sensible floor in Britain. A week is comfortable.
160W of panelwith a 150 Ah battery at 12V
- Consumption plus 20%
- 353 Wh a day
- Battery
- 150 Ah
- Charge controller
- 20 A MPPT
- Same load in December
- 590 W of panel
A system sized on the annual average will disappoint you every December, which is exactly when the nights are longest and a power cut is most likely. The December figure above is the honest one for a winter-critical system.
Panels are the cheap part. Doubling panel area costs far less than doubling battery capacity, and it is usually the better answer for a British winter.
Never charge a LiFePO4 bank below 0°C. The damage is permanent and invisible, and an unheated garage in February is below freezing at the exact hour the panel starts producing.
How this is worked out
Load first, then battery, then panel. Consumption is taken with the 20% margin the page insists on, because you will find loads you did not think of.
Battery = daily Wh x days of autonomy, divided by the usable fraction (LiFePO4 80%, Lead acid 50%), then divided by system voltage.
Panel = daily Wh divided by (peak sun hours x 0.75). The 0.75 covers wiring loss, controller loss, heat and dirt.
350 Wh a day, 4 days, LiFePO4, annual average: battery about 150 Ah, panel about 155 W. The same load in December wants about 580 W.
UK peak sun hours: June 5.5, annual average 2.0 to 3.5, December 0.8. A swing of close to seven to one, which is the number that ruins naive solar plans here.
Every figure here comes from Size and wire a 12V solar system, which gives the reasoning and the sources.